A hybrid structure employed in a construction machine such as a power shovel generates power by rotating a power generator using excess power from an engine, stores the generated power in a battery, and operates an actuator by driving an electric motor using power from the battery, for example. Further, the hybrid structure generates power by rotating the power generator using discharged energy from the actuator, stores the generated power in the battery, and operates the actuator by driving the electric motor using power from the battery (see JP2002-275945A).
In a power shovel or the like, the engine is maintained in a rotating condition even when the actuator is stopped. At such times, a pump rotates together with the engine, and therefore the pump discharges a so-called standby flow.